Conventionally, an optical disk device that reads and records information by means of light by using an optical disk such as a CD, a CD-ROM, a CD-R, a DVD, a PD, an MO or an MD as an information recording medium is known as a kind of an optical information storage device. Importance is attached to such an optical disk device because the optical disks are characterized by replaceability, large capacity, small size and lightweight. A small and low-cost laser diode is generally used as a light source of the optical information storage device represented by such an optical disk device. As the laser diode emits diffused pencils of light, there are many cases where the optical information storage device has a collimator optical system built therein for the sake of converting the diffused pencils of light to parallel pencils of light.
Presently, a large-capacity optical information storage device is under development at a rapid rate as a personal recording device of high reliability for future digital videos and broadband networks. Rendering a light source wavelength of the optical information storage device shorter and rendering an objective lens forming a focusing spot on the information recording medium higher-NA (numerical aperture) are helpful answers for the sake of reducing a diameter of the focusing spot used for recording and reproduction to realize larger capacity. If rendered shorter-wavelength and higher-NA, however, aberrations accompanying a change in ambient temperature and a switch of laser output become larger and optical stability is reduced.
Conventionally, there are some proposed optical systems having improved the optical stability against the change in ambient temperature (refer to patent documents 1, 2, 3 and 4 indicated below for instance).    (Patent Document 1)    Japanese Patent Laid-Open No. 59-15204    (Patent Document 2)    Japanese Patent Laid-Open No. 8-43702    (Patent Document 3)    Japanese Patent Laid-Open No. 11-194289    (Patent Document 4)    Japanese Patent Laid-Open No. 2002-14269
As for an optical information recording device represented by the optical disk device and the collimator optical system built into the optical information recording device, however, it is necessary to consider a change in the light source wavelength and a change in focal length of the lens as changes in conjunction with a temperature change. In addition, it is necessary to cope with the change in the light source wavelength caused by the switch of laser output when there is no temperature change. Furthermore, it is essential for the collimator optical system of the optical disk device to become smaller, and so simplification of structure and easiness of assembly must also be considered.